We have concentrated much on our effort on the interactions of steroids with the genome of neural and pituitary cells, a mechanism whereby steroids appear to affect virtually all of their target tissues. Such a mechanism of hormone action is no doubt responsible for the long-term adaptation of the neuroendocrine system to the environment. In particular, we hope to understand in terms of cellular events and chemical changes how steroid hormones bring about such adaptations as the appearance of ovulation and sexual behavior at appropriate times in adult mammals, and also how they organize the brain's ability to regulate these functions by acting during a critical period of brain development. Based upon our experience with estradiol, which appears to exert both these effects in the rat, we suspect that steroid actions on the adult brain may involve reversible alterations in the formation of gene products, such as the protein components of the neurotransmitter and neuropeptide receptors, as well as enzymes which form and inactivate neurotransmitters. On the other hand, the developmental effects of steroids, such as estradiol, may involve setting limits for gene expression later in life. These effects may take the form of permanent differences in brain circuitry, levels of steroid receptors, of inducibility of gene products by steroids and other agents.